Silver halide photography is superior in photographic characteristics, e.g., photographic speed, facility of graident control, etc., to other photographic techniques such as electrophotography, diazo photography, and so on. Therefore it has so far been employed most prevailingly. In recent years, the art has developed of forming images simply and rapidly by changing the image-forming processing used in silver halide-using photography from the conventional wet process using a developing solution or the like to a dry process using a heat-applying means or the like.
Heat-developable photosensitive materials are well-known in the photographic art, and such materials and image-forming processes using them are described, e.g., in Shashin Kogaku no Kiso, pp. 553-555, Corona Co. (1979); Eizo Joho, p. 40 (April 1978); Neblette's Handbook of Photography and Reprography, 7th Ed., pp. 32-33, Van Nostrand Reinhold Company (1977); U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020, and 3,457,075; British Pat. Nos. 1,131,108 and 1,167,777; and Research Disclosure, pp. 9-15 (RD-17029) (June 1978).
Many methods for obtaining color images through heat development have been proposed. For instance, a method of forming color images by coupling couplers to oxidation products of developers, and various kinds of developers employed therein are described in U.S. Pat. Nos. 3,531,286, 3,761,270, and 4,021,240, Belgian Pat. No. 802,519, Research Disclosure, pp. 31-32 (September 1975), and so on.
In addition, a method in which a nitrogen-containing heterocyclic group is introduced into a dye, converted into the corresponding silver salt, and made to release the dye by heat development is described in Research Disclosure, pp. 54-58 (RD-16966) (May 1978).
Moreover, a method of forming positive color images using a heat-sensitive silver dye bleach process, particularly in connection with bleaching methods of useful dyes to be employed therein, is described, e.g., in Research Disclosure, pp. 30-32 (RD-14433) (April 1976), ibid., pp. 14-15 (RD-15227) (December 1976), U.S. Pat. No. 4,235,957, and so on.
Further, method of forming color images utilizing leuco dyes is described, e.g., in U.S. Pat. Nos. 3,985,565 and 4,022,617, and so on.
However, these color image-forming methods have defects that silver halides, silver complex salts, developing agents and some ingredients remaining therein cause discoloration of the color images formed, coloration in the white background areas and so on upon long-range storage. Under these circumstances, improved methods for forming color images through heat development are proposed, e.g., in Japanese Patent Application (OPI) Nos. 179840/82, 186774/82, 198458/82, 207250/82, 58543/83, 79247/83, 116537/83, 149046/83, 48764/84, 65839/84, 71046/84, 87450/84, 88730/84, and so on.
Those methods consist in the production of the release of diffusible dyes in correspondence or countercorrespondence to the reduction of light-sensitive silver halides and/or organic silver salts to silver through heat development, and in the transfer of the resulting diffusible dyes into a dye-fixing element.
In such methods, light-sensitive silver halides and organic silver salt oxidizers are generally used in combination, as described above. On the other hand, as a heat-developable system in which silver halides alone are employed without combined with any organic silver salts to act as oxidizing agent, there have been known only those using silver chloroiodide, silver iodobromide, and silver chloroiodobromide, respectively, which contain iodide in such a condition as to demonstrate the X-ray diffraction pattern characteristic of silver iodide (for details of the system of the above-described type, Japanese Patent Application (OPI) No. 48764/84 should be referred to).
Conversely speaking, organic silver salt oxidizers have been considered essential for the system using such a silver chloride, silver bromide, silver chlorobromide, mixed crystals of silver iodide and silver chloroiodide, mixed crystals of silver iodide and silver iodobromide, and mixed crystals of silver iodide and silver chloroiodobromide (that is, no diffraction peak characteristic of silver iodide can be detected in the X-ray diffraction pattern thereof).
However, it is not easy to prepare such a silver halide mixture so as to have diffraction peaks characteristic of silver iodide crystal in its X-ray diffraction pattern. On the other hand, the combined use of silver halides and organic silver salts results in increased amounts of silver being used, tends to cause fog, tends to lower a signal to noise ratio, etc.
Therefore, it has formed a subject of great interest to find a way of effecting heat development without using any organic silver salt oxidizers in combination with silver halides, even when the silver halides have no diffraction peaks characteristic of silver iodide crystal in their X-ray diffraction pattern.